In a prior art temperature sensor, a temperature sensing element for detecting a temperature of a member to be heat-detected, e.g., a fixing roller of a fixing device is disposed between one end portion of two supporting plates made of stainless steel (SUS) for pressing said temperature sensing element against the fixing roller, said temperature sensing element and plates are fasten (sticked) together with heat-resistant films, each having an adhesive surface on one side thereof, while the other end portion of the supporting plates are secured to a holder.
In Japanese Laid-Open Patent Publication No. 1-51765, there is disclosed a toner-image fixing device comprising a resin sheet contacting with a surface of the member to be heat-detected (fixing roller), a heat-insulating elastic body for pressing said resin sheet against a surface of the fixing roller, and a temperature sensor with a temperature sensing element embedded in said heat-insulating elastic body to be pressed against said fixing roller.
The former (i.e., the temperature sensor), however, may detect a temperature lower than an actual surface temperature of a fixing roller because the supporting plates of a high thermal conductivity are cooled by air flowing therealong and the temperature sensing element contacting with the supporting plates may also be deprived a part of heat through the supporting plates.
In the latter (i.e., the toner-image fixing device), a temperature difference would appear between an actual surface temperature of the fixing roller and a detected temperature thereof, which would cause large ripples when controlling the temperature of said fixing roller to be constant, because the temperature sensing element is embedded in the heat-insulating elastic body, with a large contacting area thereof with the elastic body, whereby heat transferred to the fixing roller would be conducted to the elastic body.